Since the launch of lamivudine in China, people have become particularly concerned about hepatitis B virus mutation. More and more patients are asking about hepatitis B virus mutation, “What is hepatitis B virus mutation all about?” In fact, mutation exists in any species in nature, and it is an important way for organisms to adapt to their environment and maintain their survival, which is the law of biological evolution. However, the rate of mutation varies from species to species, and viruses are microorganisms with a relatively high mutation rate. The reasons for viral mutation include: 1. Viruses replicate at a high frequency, and the genetic material can easily mutate during the replication process; 2. Viruses replicate and reproduce in the host cells, and are inevitably attacked by the host immune system, and mutation becomes the best way to escape from immune killing. From the medical virology point of view, viral mutation constitutes a disadvantage to the treatment and prognosis of viral infectious diseases. All antiviral drugs cause viral mutation. This is because humans have to use drugs to inhibit the growth of the virus, and the virus itself has to adapt to its environment to survive, and in order to adapt to the inhibitory environment created by humans, it has to undergo some changes so that it continues to reproduce in an unfavorable environment. The site of viral mutation varies from drug to drug. Even if we do not use antiviral drugs, the virus will naturally mutate just like the flu virus, so it is common for viruses to mutate. Virus mutation occurs unknowingly and the body does not react to the mutation. However, when most viruses mutate in the body, antiviral drugs lose their inhibitory effect on the mutated virus. As a result, a relapse of the disease may occur. During the course of antiviral treatment, we ask patients to review their liver function and HBV DNA regularly. If liver function rebound occurs and HBV DNA turns positive again, it is possible that the virus has mutated. Laboratories in hospitals that are equipped to do so can detect mutated viruses by directly testing the sequence of HBV DNA.